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New Terahertz Modulator Could Lead to More Advanced Medical and Security Imaging

A UCLA Henry Samueli School of Engineering and Applied
Science research team has developed a breakthrough
broadband modulator that could eventually lead to more
advanced medical and security imaging systems.

Modulators
manipulate the intensity of electromagnetic waves. For
example, modulators in cell phones convert radio waves
into digital signals that the devices can use and
understand. In terahertz-based communication and imaging
systems, they modify the intensity of terahertz waves.

Today's
technologies take advantage of many parts of the
electromagnetic spectrum — notably light waves and radio
waves — but they rarely operate in the terahertz band,
which lies between infrared and microwave on the
spectrum.

Led by Mona
Jarrahi, UCLA associate professor of electrical
engineering, the group developed a terahertz modulator
that performs across a wide range of the terahertz band
with very high efficiency and signal clarity. Among the
device’s advantages are that it could easily be
incorporated into existing integrated circuit
manufacturing processes, can operate at room temperature
and does not require an external light source to
operate.

The
terahertz band has been the subject of extensive
research, in large part because of its potential for
medical imaging and chemical sensing technologies. For
example, terahertz waves could be used to examine human
tissue for signs for disease without damaging cells or
the other health risks posed by X-rays. They also could
be used in security screenings to penetrate fabric or
plastics that conceal weapons.

Current
optical modulators that use naturally existing
materials, such as silicon or liquid crystals, to
manipulate the intensity of light waves have proven to
be very inefficient in terahertz frequencies. And
modulators based on artificial materials, so-called
metamaterials, thus far have a limited use because they
only operate in a narrow band of the terahertz range.

The
new modulator is based on an innovative artificial
metasurface — a type of surface with unique properties
that is defined by the geometry of its individual
building blocks, and their arrangement. The metasurface
developed by Jarrahi's team is composed of an array of
micro-electromechanical units that can be opened and
closed using electric voltage. Opening or closing the
metasurface encodes the incoming terahertz wave into a
corresponding series of zeroes or ones, which are then
transformed into images.

"Our
new metasurface broadens the realm of metamaterials to
broadband operation for the first time, and it
diminishes many of the fundamental physical constraints
in routing and manipulating terahertz waves, especially
in terahertz imaging and spectroscopy systems," Jarrahi
said. "Our device geometry can switch from an array of
microscale metallic islands to an array of
interconnected metallic loops, altering its
electromagnetic properties from a transparent surface to
a reflecting surface, which manipulates the intensity of
terahertz waves passing through over a broad range of
frequencies."

The
study's lead authors are Mehmet Unlu and Mohammed Reza
Hashemi, who were postdoctoral scholars in Jarrahi's
group when she was a member of the faculty at the
University of Michigan. Other authors are Christopher
Berry and Shenglin Li, former students in Jarrahi's
group, and Shang Hua Yang, a current UCLA graduate
student.

The
research was funded by the National Science Foundation's
Sensor and Sensing Systems Division and an Army Research
Office Young Investigator award.